Field of the Invention
The present invention relates to an exposure apparatus, an exposure method and a device manufacturing method in which a substrate is, with the substrate being irradiated with an exposure light, exposed.
Description of Related Art
Semiconductor devices and liquid crystal display devices are manufactured through the so-called photolithography technique, by which a pattern formed on a mask is transferred onto a photosensitive substrate. The exposure apparatus used in the photolithography process has a mask stage that supports a mask and a substrate stage that supports a substrate, and while successively moving the mask stage and the substrate stage, transfers the mask pattern, via a projection optical system, onto the substrate. In recent years, there has been demand for higher resolution projection optical systems in order to handle the much higher levels of integration of device patterns. As the exposure wavelength to be used is shorter, the resolution of the projection optical system becomes higher. As the numerical aperture of the projection optical system is larger, the resolution of the projection optical system becomes higher. Consequently, the exposure wavelength used in exposure apparatuses has shortened year by year, and the numerical aperture of projection optical systems has also increased. Furthermore, the currently mainstream exposure wavelength is the 248 nm KrF excimer laser, but an even shorter wavelength 193 nm ArF excimer laser is also being commercialized. In addition, as well as resolution, the depth of focus (DOF) is also important when performing an exposure. The resolution R and the depth of focus δ are respectively expressed by the following formulas:R=k1·λ/NA,   (1)δ=±k2·λ/NA2,   (2)where λ the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are process coefficients. It can be seen from formulas (1) and (2) that if, to enhance the resolution R, the wavelength λ is made shorter and the numerical aperture is made larger, then the depth of focus δ becomes narrower.
When the depth of focus δ becomes too narrow, it becomes difficult to make the substrate surface coincide with the image plane of the projection optical system, and thus there occurs the possibility that the focus margin during the exposure operation will be insufficient. To address this problem, the liquid immersion method, which is disclosed in, e.g., PCT International Publication No. WO99/49504, has been proposed as a method to make the exposure wavelength shorter in effect and to make the depth of focus broader. This liquid immersion method is designed, by filling the space between the under surface of the projection optical system and the substrate surface with a liquid, e.g., water or organic solvent, to form a liquid immersion region and thus by taking advantage of the fact that the wavelength of the exposure light in the liquid becomes 1/n times (n is the refractive index of the liquid and is generally about 1.2 to 1.6) of that in the air, improve the resolution and, at the same time, enlarge the depth of focus by approximately n times.
By the way, while, in the above-described prior art, the liquid immersion region is formed on the substrate, with the supply and recovery of the liquid being performed by a liquid supply device and a liquid recovery device, there is the possibility that, when the liquid is recovered, sound and/or vibration occur, and the occurred sound and/or vibration may affect the exposure accuracy and various kinds of measurement accuracies.
It is also important that the liquid can be adequately recovered for the purposes of maintaining the exposure accuracy and/or various kinds of measurement accuracies and/or preventing the pattern formed on the substrate from deteriorating. When the liquid cannot be completely recovered, there also arises the disadvantages, for example, that the liquid remaining on the substrate dries, leaving thereon an adhesion trace (water mark), or that the remaining liquid scatters to the neighboring mechanical parts and causes them to rust. Furthermore, when the liquid remains or scatters, it may bring about, for example, the variation of the ambience (humidity, etc.) in which the substrate is placed, resulting in the change of the refractive index on the optical path of the detecting light of the optical interferometers used for the stage position measurement, thus affecting the various measurement operations associated with the exposure process, causing the exposure accuracy to be lowered.